1. Field of the Invention
The present invention relates to a battery box, and more particularly to a battery holder structure for use in a battery box to hold individual batteries securely in the battery box.
2. Description of the Related Art
Vehicles propelled by electric motors, typically electric vehicles, are generally supplied with electric energy from batteries. Usually, a plurality of batteries are housed in a battery box which is installed on the vehicle body of an electric vehicle. Since batteries, e.g., lead storage batteries, are considerably heavy, battery boxes for use in electric vehicles to hold those batteries are required to be of a rugged structure.
One conventional battery box is shown in FIG. 1 of the accompanying drawings. The battery box, generally denoted by 1 in FIG. 1, houses a total of ten batteries 2 arranged in two horizontal rows, each of five batteries, which are gripped by a pair of respective horizontal straight bars 3a, 3b each having bent opposite ends. The straight bars 3a, 3b are fastened to the bottom panel of the battery box 1 by a plurality of vertical studs 4a, 4b that are fixed to the bottom panel by nuts (not shown), thereby positioning and holding the batteries 2 securely in the battery box 1. Fastening forces applied to the studs 4a, 4b are transmitted through the straight bars 3a, 3b to the individual batteries 2 to keep the batteries 2 against accidental displacement.
FIG. 2 of the accompanying drawings illustrates a conventional battery box assembly. As shown in FIG. 2, an inner cover 6 is placed over two horizontal rows of batteries 5 and covered with a lid 7. The lid 7 is fastened to a battery box 8 by bolts and nuts (not shown). The inner cover 6 has a plurality of spaced partitions 9 positioned therein which are inserted between adjacent ones of the batteries 5 to prevent the batteries 5 from being displaced in the directions of the rows of the batteries 5.
Japanese laid-open patent publication No. 5-193367 discloses an improved battery box for use in an electric vehicle. The disclosed battery box is made of steel, and has an open top and a closed bottom on which there is mounted a reinforcing frame composed of parallel crossing members.
The improved battery box disclosed in the above publication still has some problems to be solved, as described below.
The first problem is addressed to the rigidity of the battery box. The equations (1), (2), given below, are used to calculate a maximum flexure and a maximum bending stress of a flat plate at its center when subjected to a uniformly distributed load while the flat plate is being supported at four corners thereof(see Manual of Mechanical Engineering, 6th edition, Chapter 4: Material dynamics, Table 37). ##EQU1## where .alpha.: a maximum flexure coefficient determined based on the ratio of a shorter side "a" and a longer side "b" (if b/a=2, then .alpha.=0.11);
p: the uniformly distributed load; PA1 a: the length of the shorter side PA1 E: Young's modulus; and PA1 h: the thickness of the flat plate. ##EQU2## where .beta.: a maximum stress coefficient determined based on the ratio of the shorter side "a" and the longer side "b" (if b/a=2, then .beta.=0.6); ##STR1##
According to the equation (1), the maximum flexure is proportional to the fourth power of the length of the shorter side "a" and inversely proportional to the cube of the thickness "h" of the flat plate. The length of the shorter side "a" is not of a numerical value that can easily be varied because it is determined by the number and dimensions of batteries housed. To keep the flexure within an allowable flexure range, the thickness of the bottom panel of the battery box may be increased or the thickness of the reinforcing frame may be increased.
The reinforcing frame disclosed in the above publication has a small height in its cross section, and hence has a small geometrical moment of inertia and a small modulus of section in the direction of its height. Thus, the reinforcing frame has low bending rigidity and flexural rigidity against downward loads.
As a consequence, the battery box with the reinforcing frame requires that the thickness of the bottom panel thereof and also the thickness of the reinforcing frame be increased. The resultant battery box is disadvantageous in that it is heavy.
The second problem is concerned with processes of manufacturing the battery box. The battery box disclosed in the above publication is manufactured by either a build-up process for welding four side plates to a bottom panel or a drawing process for drawing a flat plate. The build-up process is highly costly and does not lend itself to mass production. The drawing process needs an expensive drawing die, and requires corners made up of bottom and side plates to be rounded. Consequently, the drawing process fails to produce sharp corners on the battery box, which is necessarily large in size.
The third problem is directed to thermal insulation of the battery box. As disclosed in the above publication, the conventional battery box has battery cooling fans, i.e., a forced-draft fan and an induced-draft fan, for cooling the batteries only.
It has been found that the heat of the batteries includes a heat flow radiated from the batteries outwardly through the box walls and a heat flow supplied from an external source inwardly through the box walls to the batteries, and that these heat flows are responsible for temperature differences between the batteries, adversely affecting the performance and service life of the batteries.
The conventional batteries 2 shown in FIG. 1, which are typically lead storage batteries, have vertical dimensions which differ from each other within a few millimeters. Therefore, the straight bars 3a, 3b are not effective to fasten those of the batteries 2 which are not tall enough to be held in contact with the straight bars 3a, 3b. While the electric vehicle is running, those shorter batteries 2 tend to be displaced into collision with adjacent batteries 2.
Even though the inner cover 6 and the partitions 9 are added as shown in FIG. 2, the batteries 5 are not accurately positioned because the lid 7 and the battery box 8 are directly fastened to each other. Furthermore, since the rows of batteries 5 are slightly spaced from each other, they tend to move and become damaged while the electric vehicle is running.